U.S. Pat. No. 4,576,367, issued Mar. 18, 1986, to Horn et al. and assigned to the assignee of the present invention illustrates a pneumatically operated clamp of the type for use in industrial applications and for use, for example, in securing a workpiece in place during a machining, grinding, fabrication, assembly or welding operation. Such clamps generally include a clamp member automatically moveable between a workpiece clamping position and a release position. They also include a pneumatic cylinder for causing such movement of the clamp member and for maintaining a clamping force on the clamp member when the clamp member is in a clamping position. Similar apparatus are used in conveying assemblies as gates for selectively stopping movement of conveyed articles. Attention is also directed to prior art apparatus for selectively locking or controlling fluid flow in a hydraulic or pneumatic circuit. Such apparatus is illustrated in the Kepner U.S. Pat. No. 2,959,188, issued Nov. 8, 1960; the Kepner U.S. Pat. No. 3,335,750, issued Aug. 15, 1967; Kaetterhenry U.S. Pat. No. 4,018,136, issued Apr. 19, 1977; and the Panis U.S. Pat. No. 3,975,987, issued Aug. 24, 1976. Attention is also directed to the Gerulis U.S. Pat. No. 4,192,338, issued Mar. 11, 1980; the Bobnar U.S. Pat. No. 4,172,582, issued Oct. 30, 1979; the Bitonti U.S. Pat. No. 3,980,336, issued Sept. 14, 1976; the Martin U.S. Pat. No. 3,817,154, issued June 18, 1974; the Parrett et al U.S. Pat. No. 3,792,715, issued Feb. 19, 1974; the Reith U.S. Pat. No. 4,531,449, issued July 30, 1985; and the Bingel U.S. Pat. No. 3,229,721, issued Jan. 18, 1966.
Prior art locking valve cartridges for use in connection with a fluid cylinder and for preventing discharge of fluid pressure from the cylinder are illustrated in a catalog titled Hydraulic and Pneumatic Check and Relief Check Valves, issued by Kepner Products Company, Villa Park, Ill.